The present invention relates to water desalting plants and more particularly to paired stage flash evaporators used in such plants.
In multistage flash evaporator water desalting plants, brine flows through successive evaporation stages at succesively lower pressures. In each evaporation stage, water flashes to vapor which rises to an associated condenser stage where water condenses on condenser tubes and falls to a collection tray. The term "unit" as used herein is meant to refer to that portion of each multistage evaporator vessel which is associated with a single tube bundle.
In the common long flow design for multistage flash evaporator units, the brine flows from evaporation stage to evaporation stage parallel to the direction of condenser tubes which extend between the ends of the evaporator unit. A plurality of stages, for example six to eight, may be included in each unit with a common partition separating the evaporation chamber and condenser chamber of each stage from the evaporation chamber and condenser chamber of the next stage. A pair of end tube plates and a pair of water boxes are needed for each long flow unit.
While the long flow design provides economy in the total number of end tube plates and water boxes needed for a plant, it has maintenance cost disadvantages particularly those resulting from the use of lengthy condenser tubes which may span 6 to 8 stages in a single unit. The long flow design characteristics usually make it most economical from manufacturing and operating standpoints for smaller capacity plants (say up to 3 million gallons per day) and for the very high capacity plants (say 10 million gallons per day and above).
The cross-flow design is another common design for multistage flash evaporator units. In the cross-flow arrangement, each stage is provided with a pair of water boxes and a pair of end tube plates. The condenser tubes extend between the end tube plates and are associated with a single evaporation chamber in which brine flows in the direction transverse to the condenser tubes. In this arrangement, there is no internal partitioning between successive stages in each evaporator unit because each unit typically contains a single stage. Partitioning between stages is thus provided by the separation of stages between successive units.
While the cross-flow design facilitates plant operation and provides some operating economy with the use of relatively short condenser tubes in each unit, more units are required. Since each unit requires two end tube plates and two water boxes, manufacturing costs are significantly increased. The prior art cross-flow design is usually more economical in higher capacity plants (say 21/2 to 6 million gallons per day).
A more recent improved type of evaporator unit is referred to as a paired stage cross flow unit and it is disclosed in the cross-referenced patent application. In the paired stage cross-flow unit, a pair of evaporation stages and a pair of condenser stages are included in a single cross-flow unit with separation between the two evaporation chambers in the two successive evaporation stages provided by one partition which extends parallel to the tube axis and separation between the two condenser chambers provided by another partition which extends perpendicular to the tube axis direction. Flow communication is provided between the commonly staged evaporation and condenser chambers to provide for vapor flow for condensation and distillate collection. Likeness to the long flow design exists because the condenser tubes pass through two condenser chambers within a single evaporator unit. Cross-flow likeness exists because the brine flows transversely rather than longitudinally of the condenser tubes.
Because of the improved structure of the paired stage evaporator, two stages of flashing and condensing are provided in a single installed evaporator unit, where only one was possible in previous cross-flow designs. As a result, paired stage evaporator vessels provide generally improved operating efficiency, are shorter and lighter, and can be the same width as, or slightly wider than, evaporator vessels of conventional cross-flow design, for a given number of stages.
The paired stage evaporator plant, which comprises a group of adjacent evaporator units having their tube bundles connected by crossover piping, is typically smaller in plan area by 20 to 25 percent. A desalination plant of high capacity and conventional design might include 24 evaporator units connected in series with crossover piping to form an evaporator with 24 stages. In the paired stage configuration, only 12 evaporator units are needed to provide 24 stages of evaporation; the twelve paired-stage units are capable of producing the same quantity of distillate as 24 conventional cross-flow units.
As a result of the paired stage configuration, reduced tube plate and water box requirements lead to higher plant availability and lower maintenance cost and lower pumping energy costs. Single train plants can be constructed with lower initial cost and with greater total brine processing capacity.
In a conventional crossflow multistage flash evaporator, the vapors that are flashed off in each stage are passed through mesh separators and then condensed on tubes in the same stage. The distillate formed is cascaded from stage to stage in an accumulation duct and leaves the system at the lowest temperature stage.
The typical prior arrangement handles the stage-to-stage distillate cascade by using a collection duct located at one of the evaporator vessel end walls (i.e., at one end of the condenser tube bundle). In applying this arrangement to the paired-stage crossflow evaporator, two collection ducts are required for the two stages (one duct located at each end of the condenser tube bundle). The prior collection duct arrangement in the paired stage evaporator has the following disadvantages:
1. Reduced cycle thermal efficiency, since cascaded distillate is flashed off in the second stage downstream from the stage in which it is collected, thereby reducing the heat recovered in recycle condensers.
2. Increased evaporator shell cost due to the use of two collection ducts.